Organo-Lewis acid as cocatalyst for cationic homogeneous Ziegler-Natta olefin polymerizations

ABSTRACT

Organo-Lewis acids of the formula BR′R″ 2  wherein B is boron, R′ is fluorinated biphenyl, and R″ is a fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused ring group, and cationic metallocene complexes formed therewith. Such complexes are useful as polymerization catalysts.

REFERENCE TO RELATED APPLICATIONS

This application is a division of prior application Ser. No. 09/329,431, filed Jun. 10, 1999, U.S. Pat. No. 6,274,752, which is a continuation-in-part of application Ser. No. 09/220,741, filed Dec. 23, 1998, now U.S. Pat. No. 6,087,460, issued Jul. 11, 2000, which is a division of application No. 08/800,548, filed Feb. 18, 1997, now U.S. Pat. No. 5,856,256, issued Jan. 5, 1999, which in turn claims priority of U.S. provisional application Ser. No. 60/011,920, filed Feb. 20, 1996.

This invention was made with Government support under Contract No. DE-FG02-86ER13511 awarded by the Department of Energy. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

This invention relates to the compositions of matter useful as catalysts, to a method for preparing these catalysts and to a method for polymerization utilizing the catalysts.

The use of soluble Ziegler-Natta type catalysts in the polymerization of olefins is well known in the prior art. In general, such systems include a Group IV-B metal compound and a metal or metalloid alkyl cocatalyst, such as aluminum alkyl cocatalyst. More broadly, it may be said to include a mixture of a Group I-III metal alkyl and a transition metal complex from Group IVB-VB metals, particularly titanium, zirconium, or hafnium with aluminum alkyl cocatalysts.

First generation cocatalyst systems for homogeneous metallocene Ziegler-Natta olefin polymerization, alkylaluminum chlorides (AlR₂Cl), exhibit low ethylene polymerization activity levels and no propylene polymerization activity. Second generation cocatalyst systems, utilizing methyl aluminoxane (MAO), raise activities by several orders of magnitude. In practice however, a large stoichiometric excess of MAO over catalyst ranging from several hundred to ten thousand must be employed to have good activities and stereoselectivities. Moreover, it has not been possible to isolate characterizable metallocene active species using MAO. The third generation of cocatalyst, B(C₆F₅)₃, proves to be far more efficient while utilizing a 1:1 catalyst-cocatalyst ratio. Although active catalyst species generated with B(C₆F₅)₃, are isolable and characterizable, the anion MeB(C₆F₅)₃ ^(⊖), formed after Me^(⊖) abstraction from metallocene dimethyl complexes is weakly coordinated to the electron-deficient metal center, thus resulting in a drop of certain catalytic activities. The recently developed B(C₆F₅)₄ ^(⊖) type of non-coordinating anion exhibits some of the highest reported catalytic activities, but such catalysts have proven difficult to obtain in the pure state due to poor thermal stability and poor crystallizability, which is crucial for long-lived catalysts and for understanding the role of true catalytic species in the catalysis for the future catalyst design. Synthetically, it also takes two more steps to prepare such an anion than for the neutral organo-Lewis acid.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the subject invention to prepare and utilize a new class of olefin polymerization catalysts.

A further object of the subject invention is a catalyst which permits better control over molecular weight, molecular distribution, stereoselectivity, and comonomer incorporation.

Another object of the subject invention is a Ziegler-Natta type catalyst system which reduces the use of excess cocatalyst and activates previously unresponsive metallocenes.

These and other objects are attained by the subject invention whereby in one embodiment, a strong organo-Lewis acid, such as perfluorobiphenylborane (PBB) is utilized as a highly efficient cocatalyst for metallocene-mediated olefin polymerization and as a catalyst for a ring opening polymerization of THF. PBB can be synthesized in much higher yield than B(C₆F₅)₃ and the anion generated with PBB is non-coordinating instead of weakly coordinating as in the case of B(C₆F₅)₃. Thus, the former exhibits higher catalytic activities and can activate previously unresponsive metallocenes. The catalytically active species generated with PBB are isolable, X-ray crystallographically characterizable instead of the unstable, oily residues often resulting in the case of B(C₆F₅)₄ ^(⊖). In addition, PBB exhibits even higher catalytic activities in most cases.

In one embodiment of the subject invention a strong organo-Lewis acid, such as perfluorobiphenylborane (PBB), is utilized to synthesize stoichiometrically precise, isolable/crystallographically characterizable, highly active “cation-like” metallocene polymerization catalysts. The biphenyl groups of PBB may be connected to the boron at the meta, para, or ortho position.

PBB reacts with early transition metal or actinide alkyls to yield highly reactive cationic complexes: (CpCp′MR)^(⊕)(RBR′R″₂)^(⊖)

where

CpCp′=C₅H_(n)R_(5-n)(n is 0-5), indenyl, allyl, benzyl, C₅H_(n)R_(4-n)XNR (n is 0-4);

M=early transition metal or actinide, e.g., Ti, Zr, Hf, Th, U;

X=R′″₂Si, where R′″ is an alkyl or aryl group (C≦10);

R, R′″=alkyl, benzyl, or aryl group (C≦20), hydride, silyl;

B=boron

R′=fluorinated biphenyl

R″=fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused rings such as naphthyl, anthracenyl, or fluorenyl.

As a specific example of the above, the reaction of PBB with a variety of zirconocene dimethyl complexes proceeds rapidly and quantitatively to yield, after recrystallization from hydrocarbon solvents, the catalytic complex of Eq. 1.

Such catalytic complexes have been found to be active homogeneous catalysts for α-olefin polymerization and, more particularly, the polymerization, copolymerization or oligopolymerization of ethylene, α-olefins, dienes and acetylenic monomers, as well as intramolecular C—H activation.

The cocatalyst of the subject invention may be referred to as BR′R″₂, where B=boron; R′ and R″ represent at least one and maybe more fluorinated biphenyls or other polycyclic groups, such as naphthyl. Two of the biphenyls may be substituted with a phenyl group. Both the biphenyls and the phenyl groups should be highly fluorinated, preferably with only one or two hydrogens on a group, and most preferably, as in PBB with no hydrogens and all fluorines.

BRIEF DESCRIPTION OF THE DRAWINGS

The cocatalyst system of the subject invention can be better understood with reference to the drawings wherein:

FIG. 1 is a structural depiction of PBB;

FIG. 2 is a reaction pathway for the synthesis of PBB;

FIG. 3 shows the reaction pathway for a catalyst system according to the subject invention;

FIG. 4 shows the reaction pathway for a second catalyst system according to the subject invention;

FIG. 5 shows the reaction pathway for a third catalyst system according to the subject invention;

FIG. 6 shows the reaction pathway for a fourth catalyst system according to the subject invention; and

FIG. 7 shows the reaction pathway for a fifth catalyst system according to the subject invention.

DETAILED DESCRIPTION OF THE INVENTION

The reaction of perfluorobiphenylborane with a variety of zirconocene and other actinide or transition metal dimethyl complexes proceeds rapidly and quantitatively at room temperature in noncoordinating solvents to yield, after recrystallization, complexes. This catalyst activation reaction may be used in the polymerization, copolymerization, oligomerization and dimerization of α-olefins. In addition, the catalyst of the subject invention may be used in conjunction with aluminum alkyls, aluminum aryls, (AlR₃, R=Et, Me, Ph, naphthyl) or methyl alumoxane (Al(CH₃)O)_(n) for increased polymer yields.

PBB (FIG. 1) has been synthesized in quantitative yields of 91% as compared to the 30-50% yields experienced with B(C₆F₅)₃, currently a very important Lewis acidic cocatalyst in industry (FIG. 2). The Lewis acidity of PBB has been shown to be much greater than that of B(C₆F₅)₃ by comparative reactions of Cp*₂ThMe₂ with B(C₆F₅)₃ and PBB (Cp*=C₅Me₅). The former reagent does not effect Me^(⊖) abstraction, while the latter gives the catalyst shown in FIG. 3. The reaction of PBB with a bis-Cp type of dimethyl zirconocenes forms a dinuclear methyl-bridged zirconocene cation such as

(1:1 or 2:1)

where

Cp=C₅H₅

Cp=C₅H₃Me₂ or

Cp=C₅Me₅

and a hydride-bridged analog such as

where

Cp=C₅H₅ or

Cp=C₅H₃Me₂

More particularly, reaction of PBB with group 4 and Th methyls proceeds cleanly to yield cationic complexes such as set forth below.

Cp = η⁵-C₅H₅ 1, Cp*₂ThMe^(⊕)[MePBB]^(⊖) Cp* = η⁵-Me₅C₅ 2, Cp₂ZrCl^(⊕)[MePBB]^(⊖) Cp″ = η⁵-1,2-Me₂C₅H₃ 3, [Cp₂ZrMe(μ-Me)MeZrCp₂]^(⊕)[MePBB]^(⊖) CGC = (Me₄C₅)SiMe₂N^(t)Bu 4, [Cp″₂ZrMe(μ-Me)MeZrCp″₂]^(⊕)[MePBB]^(⊖) M = Th, Zr, Hf, Ti 5, [Cp₂*ZrMe(μ-Me)MeZrCp₂*]^(⊕)[MePBB]^(⊖) 6, [(Me₄C₅)SiMe₂N^(t)Bu]ZrMe^(⊕)[MePBB]^(⊖) 7, [(Me₄C₅)SiMe₂N^(t)Bu]TiMe^(⊕)[MePBB]^(⊖) 8, Cp*ZrMe₂ ^(⊕)[MePBB]^(⊖) 9, Cp*HfMe₂ ^(⊕)[MePBB]^(⊖)

For ethylene polymerization, catalytic activities of dinuclear cations generated from PBB are greater than those of monomeric cations generated from B(C₆F₅)₃ presumably because (MePBB)^(⊖) is a non-coordinating anion as compared to the weakly coordinating anion MeB(C₆F₅)₃. The dinuclear cations have also been found to catalyze the rapid ring-opening polymerization of THF to produce poly(tetrahydrofuran), an important thermoplastic elastomer and artificial leather. Monomeric zirconocene cations have also been generated in situ by the reaction of Cp₂ZrMe₂ and PBB at 60° C.:

These attempts show very high activities for olefin polymerization, and identify (MePBB)^(⊖) to be a truly non-coordinating anion. The polymerization data with metallocene cations having various anions are summarized in Table 1.

TABLE 1 Polymerization Data Entry μmol Polymer M_(wd) ^(c) No. Catalyst of cat Conditions Monomer(s)^(a) yield (g) Activity^(b) (10⁻³) M_(w)/M_(n) Remarks 1 (Cp₂ZrMe)₂Me^(⊕) 15 100 mL toluene ethylene 0.80 4.80 × 10⁶ 559 3.06 MePBB^(⊖) 25° C., 40 s 2 Cp₂ZrMe^(⊕) 15 100 mL toluene ethylene 1.00 4.00 × 10⁶ 124 2.03 MeB(C₆F₅)₃ ^(⊖) 25° C., 60 s 3 (Cp″₂ZrMe)₂Me^(⊕) 15 100 mL toluene ethylene 1.30 7.80 × 10⁶ 392 2.72 MePBB^(⊖) 25° C., 40s 4 Cp″₂ZrMe^(⊕) 15 100 mL toluene ethylene 1.50 6.00 × 10⁶ 321 1.42 MeB(C₆F₅)₃ ^(⊖) 25° C., 60 s 5 (Cp*₂ZrMe)₂Me^(⊕) 15 100 mL toluene ethylene 1.07 4.30 × 10⁶ 370 2.28 MePBB^(⊖) 25° C., 60 s 6 Cp*₂ZrMe^(⊕) 15 100 mL toluene ethylene 0.80 3.20 × 10⁶ 136 2.54 MeB(C₆F₅)₃ ^(⊖) 25° C., 60 s 7 Cp*TiMe^(⊕) ₂ 50 5 mL toluene styrene 0.35 1.61 × 10⁶ 170 2.56 [rrrr] > 98% MePBB^(⊖) 25° C., 15 min 8 Cp*ZrMe^(⊕) ₂ 50 5 mL toluene styrene 1.45 1.00 × 10⁷ 27.6 2.63 atactic MePBB^(⊖) 25° C., 10 min 9 Cp*HfMe^(⊕) ₂ 50 5 mL toluene styrene 0.69 3.17 × 10⁶ 24.8 2.98 atactic MeB(C6F)₃ ^(⊖) 25° C., 15 min 10 Cp*HfMe^(⊕) ₂ 50 5 mL toluene styrene 1.16 5.33 × 10⁶ 22.9 2.78 atactic MePBB^(⊖) 25° C., 15 min 11 Cp*TiMe^(⊕) ₂ 50 25 mL toluene ethylene 0.70 1.70 × 10⁵ 848 23.7 39.5% hexene MeB(C₆F₅)₃ ^(⊖) 25° C., 5 min 1-hexene incorporation 12 Cp*TiMe^(⊕) ₂ 50 25 mL toluene ethylene 4.51 1.08 × 10⁶ 151 4.32 43.6% hexene MePBB^(⊖) 25° C., 5 min 1-hexene incorporation 13 CGCZrMe^(⊕) 15 100 mL toluene ethylene 0 — — — MeB(C₆F₅)₃ ^(⊖) 25° C., 20 min 14 CGCZrMe^(⊕) 15 100 mL toluene ethylene 1.56 1.56 × 10⁶ 7.69 2.78 MePBB^(⊖) 25° C., 4 min 15 CGCTiMe^(⊕) 15 100 mL toluene ethylene 0.21 8.40 × 10⁴ 1058 9.54 MeB(C₆F₅)₃ ^(⊖) 25° C., 10 min 16 CGCTiMe^(⊕) 15 100 mL toluene ethylene 0.83 4.98 × 10⁶ 305 2.56 MePBB^(⊖) 25° C., 40 s 17 CGCZrMe^(⊕) 50 25 mL toluene ethylene 0 — — — MeB(C₆F₅)₃ ^(⊖) 25° C., 15 min 1-hexene 18 CGCZrMe^(⊕) 50 25 mL toluene ethylene 6.97 5.58 × 10⁵ 10.0 2.68 33.6% hexene MePBB^(⊖) 25° C., 15 min 1-hexene incorporation 19 CGCTiMe^(⊕) 25 25 mL toluene ethylene 0.05 1.20 × 10⁴ 63.2% hexene MeB(C₆F₅)₃ ^(⊖) 25° C., 10 min 1-hexene incorporation 20 CGCTiMe^(⊕) 25 25 mL toluene ethylene 1.95 4.68 × 10⁵ 105 1.86 65.3% hexene MePBB^(⊖) 25° C., 10 min 1-hexene incorporation ^(a)1 atm ethylene pressure; 17.4 mmol of styrene, & 44.5 mmol of 1-hexene. ^(b)g polymer/[(mol of cationic metallocene) · atm· h], except in entries 7-10: polystyrene/[(mol catalyst) · (mol monomer) · h] (reproducibility between runs ≈ 10˜15%). ^(c)GPC relative to polystyrene standards.

Other types of cationic metallocene catalyst systems can also be created with PBB. Metallocene cations of mono-Cp type (FIGS. 4 and 5) have been formed by the reaction of mono-pentamethyl Cp trimethyl group IV complexes with PBB. These are very good syndiospecific styrene polymerization catalysts. Constrained geometry types of zirconocene and titanocene cations such as those in FIG. 6 where m=Zr, Ti, are readily produced by the reaction of the corresponding dimethyl metallocenes with PBB. They are highly naked cations and much more active catalysts than those generated with B(C₆F₅)₃.

EXAMPLE 1 Synthesis of Perfluorobiphenylborane (PBB)

n-Butyllithium (1.6 M in hexanes, 25 mL, 40 mmol) was added dropwise to bromopentafluorobenzene 18.0 g, 9.1 mL, 72.9 mmol) in 100 mL of diethyl ether over a cold-water bath. The mixture was then stirred for a further 12 h at room temperature. Removal of solvent followed by vacuum sublimation at 60-65° C./10⁻⁴ torr gave 12.0 g of 2-bromononafluorobiphenyl as a white crystalline solid: yield 83.3%. The dangerous and explosive nature of C₆F₅Li-ether solutions in this preparation can be avoided by (a) the use of excess of C₆F₅Br, (b) slow addition of n-butyllithium, (c) frequent change of the cold water bath, or use of a continuous flowing cold water bath.

To the above prepared 2-bromononafluorobiphenyl (5.0 g, 12.7 mmol) in a mixed solvent of 70 mL of diethyl ether and 70 mL of pentane was gradually added 8.0 mL of n-butyllithium (1.6 M in hexanes, 12.8 mmol) at −78° C. The mixture was stirred for an additional 2 h, and boron trichloride (4.0 mL, 1.0 M in hexanes, 4.0 mmol) was then quickly added by a syringe. The mixture was left at −78° C. for 1 h and the temperature was then allowed to slowly rise to room temperature. A suspension resulted after stirring an additional 12 h. It was filtered to give a yellow solution, and the solvent of the filtrate was removed in vacuo. The resulting pale yellow powder was sublimed at 140° C./10⁻⁴ torr or 125° C./10⁻⁶ torr to produce a light yellow crystalline solid as an ether-free crude product. Recrystallization from pentane at −20° C. gave 3.5 g of the pure PBB as a white crystalline solid: yield 91.0%. Analytical and spectroscopic data for PBB are as follows. ¹⁹F NMR (C₆D₆, 23° C.): δ-120.08 (s, br, 3 F, F-3), −132.09 (s, br, 3 F, F-6), −137.66 (s, br, 6 F, F-2′/F-6′), −143.31 (t, ³J_(F-F)=21.4 Hz, 3 F, F-4), −149.19 (t, ³J_(F-F)=21.7 Hz, 3 F. F-4′), −150.56 (t, ³J_(F-F =)14.7 Hz, 3 F, F-5), 160.72 (s, br, 6 F, F-3′/F-5′). ¹³C NMR(C₆D₆, 23° C.): δ150.92 (dd, ¹J_(C-F)=251.8 Hz, ²J_(C-F)=10.1 Hz, 3 C),146.35 (dd, ¹J_(C-F)=254.3 Hz, ²J_(C-F)=12.1 Hz, 3 C), 144.26 (dd, ¹J_(C-F)=258.1 Hz, ²J_(C-F)=10.5 Hz, 6 C), 143.50 (tt, ¹J_(C-F)=265.4 Hz, ²J_(C-F)=12.0 Hz, 3 C), 141.98 (tt, ¹J_(C-F)=261.4 Hz, ²J_(C-F)=11.7 Hz, 3 C), 141.17 (tt, ¹J_(C-F)=254.3 Hz, ²J_(C-F)=10.5 Hz, 3 C), 137.70 (tt, ¹J_(C-F)=257.3 Hz, ²J_(C-F)=11.6 Hz, 6 C), 124.51 (d, ²J_(C-F)=11.7 Hz, 3 C), 113.60 (d, ²J_(C-F)=11.5 Hz, 3 C), 106.05 (s, br, 3 C). MS: parent ion at m/e 956. Anal. Calcd for C₃₆BF₂₇: C, 45.22; H, 0.00. Found: C, 45.44; H, 0.05.

EXAMPLE 2 Synthesis of Cp*₂ThMe^(⊕)(MePBB)^(⊖)

Cp*₂ThMe₂ (0.106 g, 0.199 mmol) and PBB (0.191 g, 0.199 mmol) were in the glove box charged into a 25-mL reaction flask with a filter plug, and the flask was attached to the high vacuum line. Benzene (15 mL) was then vacuum-transferred into this flask at −78° C. The mixture was slowly allowed to warm to room temperature and stirred for 6 h. The solvent was removed, pentane (20 mL) was next vacuum-transferred into the flask, and the mixture was filtered after stirring. The white solid which collected was dried under vacuum to give 0.210 g of product: yield 70.9%. Analytical and spectroscopic data are as follows. ¹H NMR (C₆D₆, 23° C.): δ1.61 (s, 30 H, C₅Me₅), 0.62 (s, 3 H, Th—CH₃), −095 (s, br, 3 H, B—CH₃). ¹⁹F NMR (C₆D₆, 23° C.): δ-124.57 (s, br, 3F), −138.10 (s, br, 3 F), −139.28 (d, ³J_(F-F)=21.4 Hz, 3 F), −139.74 (d, ³J_(F-F)=21.2 Hz, 3 F), −155.08 (t, ³J_(F-F) 21.4 Hz, 3 F), −157.32 (t, ³J_(F-F)=22.0 Hz, 3 F), −162.20 (t, ³J_(F-F)=22.0 Hz, 3 F), −163.13 (t, ³J_(F-F)=22.0 Hz, 3 F), −163.90 (t, ³J_(F-F)=21.4 Hz, 3 F). ¹³C NMR (C₆D₆, 23° C.): δ129.54 (C₅Me₅), 79.28 (Th—Me), 10.44 (C₅Me₅), 10.25 (B—Me). Anal. Calcd for C₅₈ H₃₆ BF₂₇ Th: C, 46.79; H, 2.44; N, 0.00. Found: C, 46.68; H, 2.24; N. 0.00.

EXAMPLE 3 Synthesis of Cp₂Zr(Me)(μ-Me)(Me)ZrCp₂ ^(⊕)(MePBB)^(⊖)(Cp=C₅H₅, C₅H₃Me₂, or C₅Me₅

Cp₂ZrMe₂ (0.398 mmol) and PBB (0.199 mmol) were loaded into a 25 mL-flask, which was then attached to the vacuum line. Pentane (20 mL) was then vacuum-transferred into this flask at −78 ° C. The mixture was slowly warmed to room temperature and stirred for an additional 2 h (Cp=C₅H₅), 15 h (Cp=C₅H₃Me₂) or 48 h (Cp=C₅Me₅). The resulting suspension was filtered, and the colored solids (light pink for C₅H₅, light yellow for C₅H₃Me₂ and yellow for C₅Me₅) were washed with a small amount of pentane and dried under vacuum: yields 90.3% (C₅H₅), 86.3% (C₅H₃Me₂) and 34.7% (C₅Me₅). Analytical and spectroscopic data for Cp=C₅H₅ are as follows. ¹H NMR (C₆D₆, 23° C.): δ5.65 (s, 20 H, C₅H₅), −0.04 (s, 6 H, Zr—CH₃), −0.84 (s, br, 3 H, B—CH₃), −1.15 (s, 3 H, Zr—CH₃—Zr). ¹⁹F NMR (C₆D₆, 23° C.): δ124.20 (d, ³J_(F-F)=16.6 Hz, 3 F), −138.98 (d, ³J_(F-F)=20.3 Hz, 3 F), −139.20 (d, ³J_(F-F)=22.0 Hz, 3 F), −140.29 (d, ³J_(F-F)=24.5 Hz, 3 F), −155.15 (t, ³J_(F-F)=20.9 Hz, 3 F), −160.06 (t, ³J_(F-F)=22.3 Hz, 3 F), −162.79 (t, ³J_(F-F)=22.0 Hz, 3 F), −163.11 (t, ³J_(F-F)=21.5 Hz, 3 F), −163.97 (t, ³J_(F-F)=19.0 H,3 F). ¹³C NMR(C₆D₆, 23° C.):δ113.24(C₅H₅),38.88(Zr—CH₃),21.53 (B—CH₃), 15.80 (Zr—CH₃-Zr). Anal. Calcd for C₆₀H₃₂BF₂₇Zr₂: C, 49.39; H, 2.21; N, 0.00. Found: C, 48.97; H, 1.92; N 0.00.

Analytical and spectroscopic data for Cp=C₅H₃Me₂ are as follows. ¹H NMR (C₇D₈, 23° C.): δ5.51 (t, ³J_(H-H)=2.8 Hz, 4 H, C₅H₃ Me₂), 5.47 (t, ³J_(H-H)=3.2 Hz, 4 H, C₅H₃Me₂), 5.18 (t, ³J_(H-H)=2.8 Hz, 4 H, C₅H₃Me₂).1.73 (s, 12 H, C₅H₃Me₂), 1.51 (s, 12 H, C₅H₃MMe₂), −0.26 (s, 6 H, Zr—CH₃), −0.92 (s, br, 3 H, B—CH₃), −1.50 (s, 3 H, Zr—CH₃-Zr). ¹⁹F NMR (C₆D₆, 23° C.): δ123.37 (d, ³J_(F-F)=15.3 Hz, 3 F), −139.20 (d, ³J_(F-F)=24.0 Hz, 3 F), −139.62 (d, ³J_(F-F)=24.3 Hz, 3 F), −139.89 (d, ³J_(F-F)=24.0 Hz, 3 F), −155.81 (t, ³J_(F-F)=2.14 Hz, 3 F), −159.36 (t, ³J_(F-F)=22.3 Hz, 3 F), −163.22 (t, ³J_(F-F)=21.4 Hz, 3 F), −16.55 (t, ³J_(F-F)=22.0 Hz, 3 F), −164.20 (t, ³J_(F-F)=22.6 Hz, 3 F). ¹³C NMR (C₆D₆, 23° C.): δ114.20 (d, ¹J_(CH)=17.1 Hz, C₅H₃Me₂), 113.62 (s, C₅H₃Me₂), 112.80 (s, C₅H₃Me₂), 111.29 (d, ¹J_(CH)=165.7 Hz, C₅H₃Me₂), 106.57 (d, ¹J_(CH)=173.3 Hz. C₅H₃Me₂), 41.63 (q, ¹J_(CH)=118.4 Hz, Zr—CH₃), 31.26 (q, ¹J_(CH)=116.5 Hz, B—CH₃), 22.21 (q, ¹J_(CH)=134.3 Hz, Zr—CH₃-Zr), 12.94 (q, ¹J_(CH)=128.0 Hz, C₅H₂Me₂), 12.71 (q, ¹J_(CH)=127.6 Hz. C₅H₂Me₂). Anal. Calcd for C₆₈H₄₈BF₂₇Z₂: C, 51,98; H, 3.08; N, 0.00. Found: C, 51.61; H, 3.00; N, 0.00.

Analytical and spectroscopic data for Cp=C₅Me₅ are as follows. ¹H NMR (C₆D₆, 23° C.): δ1.57 (s, 60 H, C₅Me₅) −0.84 (s, br, 3 H, B—CH₃). The bridging and terminal methyl groups are discrete at low temperature. ¹H NMR (C₇D₈, −13° C.): δ−0.19 (s, br, 6 H. Zr—CH₃), −0.92 (s, br, 3 H, B—CH₃), −2.42 (s, br, 3 H, Zr—CH₃-Zr). ¹⁹F NMR (C₆D₆, 23° C.): δ−123.11 (d, s, br, 3 F), −139.27 (d, ³J_(F-F)=20.3 Hz, 3 F), −139.67 (t, ³J_(F-F)=25.1 Hz, 6F), −155.73 (t, 3J_(F-F)=20.9 Hz, 3 F), −160.91 (s, br, 3 F), −163.25 (t, ³J_(F-F)=21.7 Hz, 3F), −163.56 (t, ³J_(F-F)=22.0 Hz, 3 F), −164.13 (t, ³J_(F-F)=21.4 Hz, 3 F). Anal. Calcd for C₈₀H₇₂BF₂₇Zr₂: C, 55.23; H, 4.17; N, 0.00. Found: C, 54.81; H, 3.98; N, 0.00.

EXAMPLE 4 Synthesis of Cp₂Zr(H)(μ-H)(H)ZrCp₂ ^(⊕)(MePBB)^(⊖); Cp=C₅H₅, C₅H₃Me₂

The procedure here is similar to that of Example 3, except that the reaction was carried out under 1 atm of H₂ for 15 h: yields 81.6% (Cp=C₅H₅, grey solid) and 75.6% (Cp=C₅H₃Me₂, orange solid). Analytical and spectroscopic data for Cp=C₅H₅ are as follows. ¹H NMR (C₆D₆, 58° C.): δ6.67 (s, br, 2 H, Zr—H), 5.64 (s, 20 H, C₅H₅), −0.81 (s, br, 3 H, B—CH₃), −1.38 (s, br, 1 H, Zr—H—Zr). The chemical shifts and splitting patterns of ¹⁹F NMR are same as those of Example 3 (Cp=C₅H₅). Anal. Calcd for C₅₇H₂₆BF₂₇Zr₂: C, 48.31; H, 1.85; N, 0.00. Found: C, 47.90; H, 1.92; N, 0.00.

Analytical and spectroscopic data for Cp=C₅H₃Me₂ are as follows. ¹H NMR (C₇D₈, 23° C.): δ5.81 (m, 4 H, C₅H₃Me₂),5.50 (m,4 H, C₅H₃Me₂),523 (m, 4 H, C₅H₃Me₂). 1.65 (m, 24 H, C₅H₃Me₂), 0.25 (s, br, 2 H, Zr—H), −0.94 (s, br, 3 H, B—CH₃), −1.52 (s, br, 1 H, Zr—H—Zr). The chemical shifts and splitting patterns of ¹⁹F NMR are same as those of Example 3 (Cp=C₅H₃Me₂). Anal. Calcd for C₆₅H₄₂BF₂₇Zr₂: C, 51.05; H, 2.77; N, 0.00. Found C, 51.07; H. −2.63; N. 0.00.

EXAMPLE 5 Preparation of Cp₂ZrMe^(⊕)(MePBB)^(⊖)

5(a) Cp=C₅H₅. In a J-Young NMR tube, a small amount of a mixture of Cp₂ZrMe₂ and PBB (1:1.2 molar ratio) was dissolved in C₆D₆). The NMR tube was then put in an NMR probe and heated at 60° C. After 0.5 h, ¹H NMR revealed the above monomeric species formed. The same structures were obtained by the reaction of the product of Example 3 with excess of PBB at 60° C. for 0.5 h. In a real polymerization test, these species were also generated in situ by mixing Cp₂ZrMe₂ and PBB at 60° C. for 0.5 h. ¹H NMR (C₆D₆, 60° C.) for: δ5.70 (s, 10 H, C₅H₅), 0.14 (s, 3 H, Zr—CH₃), −0.85 (s, br, 3 H, B—CH₃). The ¹⁹F NMR is similar to that of the corresponding dinuclear species of Example 3 (Cp=C₅H₅).

5(b) Cp=C₅H₃Me₂. The same procedure of Example 5(a) was used to prepare this species. In the polymerization test, the following was observed: ¹H NMR (C₇D₈, 60° C.) for 8: δ5.68 (t, 3 J H—H=2.8 Hz, 4 H, C₅H₃Me₂), 5.36 (t, ³J_(H-H)=3.1 Hz, 4 H, C₅H₃Me₂), 5.23 (t, ³JH-H=2.8 Hz, 4 H, C₅H₃Me₂).1.76 (s, 6 H, C₅H₃Me₂), 1.56 (s, 6 H, C₅H₃Me₂), 0.17 (s, 3 H, Zr—CH₃), −0.93 (s, br, 3 H, B—CH₃). ¹⁹F NMR of this species is similar to that of the corresponding dinuclear species of Example 3 (Cp=C₅Me₅). ¹³C NMR (C₇D₈, 60° C.): δ117.74 (C₅H₃Me₂), 112.14 (C₅H₃Me₂), 108.01 (C₅H₃Me₂), 42.11 (Zr—CH₃), 34.43 (B—CH₃), 12.63 (C₅H₂Me₂), 12.45 (C₅H₂Me₂).

5(c) Cp=C₅Me₅ The same procedure of Example 5 (a) was used to prepare this species. In the polymerization test, the following was observed: ¹H NMR (C₆D₆, 60° C.): δ1.61 (s, 30 H, C₅Me₅), 0.13 (s, 3 H, Zi—CH₃), −0.86 (s, br, 3 H, B—CH₃). ¹⁹F NMR is similar to that of the corresponding dinuclear species of Example 3, Cp=C₅Me₅.

EXAMPLE 6 Synthesis of CpM(Me)₂ ^(⊕)(MePBB)^(⊖); Cp=C₅Me₅

M=Ti. The catalyst product of FIG. 5 was generated in the NMR tube reaction by mixing C₅Me₅TiMe₃ and PBB at 1:1 molar ratio in C₆D₆for2h. ¹H NMR(C₆D₆, 23° C.): δ9.03 (s, br, 2 H. CH₂), 1.69 (s, 6 H, C₅Me₄), 1.65 (s, 6 H, C₅Me₄), 0.15 (s, 3 H, Ti—CH₃), −0.82 (s, br, 3 H, B—CH₃).

EXAMPLE 7 Synthesis of Me₂Si(^(t)BuN)(C₅Me₄)MMe^(⊕)(MePBB)^(⊖)

M=Zr. Me₂Si(^(t)BuN)(C₅Me₄)MMe₂ (0.199 mmol) and PBB (0.199 mmol) were treated in the same manner as in the preparation of Example 2 except for the different reaction times (2 h). This procedure yields 73.1 % (yellow solid). Analytical and spectroscopic data are as follows. ¹H NMR (C₇D₈, 23 ° C.): δ1.73 (s, 3 H, C₅Me₄), 1.69 (s, 3 H, (C₅Me₄), 1.63 (s, 3 H, C₅Me₄), 1.43 (s, 3 H, C₅Me₄), 0.85 (s, 9 H, N-^(t)Bu), 0.28 (s, 3 H, SiMe₂), 0.21 (s, 3 H, SiMe₂), −0.48 (s, 3 H, Zr—CH₃), −0.95 (s, br, 3 H, B—CH₃). ¹⁹F NMR (C₇D₈, 23° C.): δ124.20 (s, br, 3 F), −139.14 (d, ³J_(F-F)=23.7 Hz, 3 F), −139.35 (d, ³J_(F-F)=22.0 Hz, 3 F), −139.93 (d, ³J_(F-F)=21.2 Hz, 3 F), −155.79 (t, ³J_(F-F)=21.2 Hz, 3 F), −159.67 (t, ³J_(F-F)=22.3 Hz, 3 F), −163.28 (t, ³J_(F-F)=21.7 Hz, 3 F), −163.87 (t, ³J_(F-F)=22.6 Hz, 3 F), −164.13 (t, ³J_(F-F)=22.6 Hz, 3 F). ¹³C NMR (C₇D₈, 23° C.): δ130.22 (C₅Me₄), 128.18 (C₅Me₄), 127.22 (C₅Me₄), 126.47 (C₅Me₄), 124.37 (C₅Me₄), 58.47 (N—CMe₃), 34.37 (Zr—CH₃), 34.10 (N—CMe₃), 15.89 (C₅Me₄), 13.46 (C₅Me₄), 11.77 (C₅Me₄), 10.99 (C₅Me₄), 7.92 (SiMe₂), 5.65 (SiMe₂). Anal. Calcd for C₅₃H₃₃BF₂₇NSiZr: C, 47.97; H, 2.51; N, 1.06, Found: C, 47.79; H, 2.58; N, 0.86.

EXAMPLE 8 Ethylene Polymerization

The reaction was conducted in a 250 mL flamed round bottom flask attached to a high vacuum line. The flask was equipped with a large magnetic stirring bar and a straight-bore high vacuum stopcock. The exterior connecting tube of the stopcock (Ca. 10 mm in length) is sealed with a new serum cap. The reaction vessel is then evacuated for several hours, back-filled with inert gas (Ar), the stopcock closed, and the reaction flask reevacuated. A measured amount of a nonpolar solvent such as benzene or toluene is vacuum transferred into the flask. Gaseous ethylene is admitted to the reaction flask through the purification column. The gas pressure is continuously maintained at 1 atm. Rapid stirring of the solution is initiated and after several minutes (to allow the saturation of the solvent with ethylene), the stopcock is opened and a small aliquot of catalyst solution (in the same solvent as used for the reaction) is injected by a gas-tight syringe just above the rapidly stirring solution through a serum cap (the syringe needle had been flattened so that the catalyst solution exits in a fine spray). Solid polyethylene is formed immediately. The reaction is quenched after a certain amount of time by injecting methanol through the serum cap on the stopcock. The solid polyethylene was collected by filtration, washed with methanol and then dried under vacuum at 100° C. Copolymerization may occur with the addition of a second monomer such as another α-olefin.

Ethylene polymerizations were carried out at room temperature in 250-mL flamed, round-bottom flasks attached to a high-vacuum line. In a typical experiment, a solution of each of the catalysts of Example 3 in 2 mL of toluene was quickly injected using a gas-tight syringe equipped with a spraying needle into respective rapidly stirred flasks containing 100 mL of toluene which was pre-saturated under 1 atm of rigorously purified ethylene. In the case of the catalysts prepared in Example 3, the catalyst solution was generated in situ by mixing Cp₂ZrMe₂ and PBB in 2 mL of toluene after aging for 0.5 h at 60° C., and then quickly injected into respective flasks under an ethylene atmosphere using a pre-warmed gas-tight syringe. The polymerization was quenched with acidic CH₃OH after a short time period (10-60 s) at which point voluminous quantities of polyethylene precipitated out. The respective polymeric products were collected by filtration, washed with methanol and dried under high vacuum to a constant weight.

EXAMPLE 9 Ring-Opening Polymerization of THF

A small amount of [(C₅H₃Me₂)₂(Me)Zr—Me—Zr(Me)(C₅H₃Me₂)₂]^(⊕)(MePBB)^(⊖) was loaded into a J-Young NMR tube and THF-d₈ was then vacuum-transferred into the tube. The mixture was slowly warmed to room temperature and left for several hours. The solid polymer formed in the tube was shown to be polytetrahydrofuran by ¹H analysis.

EXAMPLE 10 Propylene Polymerization

This reaction is carried out in a 100 mL quartz Worden vessel equipped with a magnetic stirring bar, a pressure gauge and a stainless steel o-ring assembly attached to a high vacuum line. In a typical experiment, the reaction vessel is flamed and then pumped under high vacuum for several hours, filled with inert gas and brought into a glove box. A measured amount of catalyst is added into the vessel. On the high vacuum line, a measured amount of the solvent and propylene are condensed in at −78 ° C. The reaction apparatus is sealed off and warmed to the desired temperature. During the polymerization process, the reaction tube is immersed in a large amount of tap water (20-25° C.) or ice water (0° C.) to help dissipate the heat produced from the polymerization and keep the temperature constant. The progress of the polymerization reactions is monitored through observance of the pressure change. After the reaction is finished (pressure drops to zero psi), the resulting oily liquid is removed from the vessel, washed with methanol and water and dried under vacuum at 90-100° C. for ten hours to result in a colorless oil.

Table II sets forth the relevant data concerning propylene polymerization utilizing the catalyst prepared according to the enumerated example.

TABLE II Example 3 5 Metallocene (Cp₂ZrMe)₂Me^(⊕)/ (Cp₂ZrMe^(⊕))/ Cation/Anion* (MePBB)^(⊖) (MePBB)^(⊖) Catalyst (mM) 0.15 0.15 Reaction Time (m) 40 40 Yield (g) 4.0 5.0 *Cp = C₅H₅

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments and equivalents falling within the scope of the appended claims.

Various features of the invention are set forth in the following claims. 

That which is claimed is:
 1. An organo-Lewis acid of the formula BR′R″₂ wherein B is boron, R′ is fluorinated biphenyl, and R″ is a fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused ring group.
 2. An organo-Lewis acid of claim 1 wherein said polycyclic fused ring groups are naphthyl, anthracenyl, or fluorenyl.
 3. An organo-Lewis acid of claim 1 wherein each R″ is a fluorinated biphenyl with (i) only one or two hydrogens thereon or (ii) no hydrogens and all fluorines thereon.
 4. An organo-Lewis acid of claim 1 wherein each R″ is a fluorinated biphenyl substituted with a phenyl group and wherein both said biphenyls and the phenyl groups have (i) only one or two hydrogens thereon or (ii) no hydrogens and all fluorines thereon.
 5. Tris(perfluorobiphenyl)borane.
 6. A solution of tris(perfluorobiphenyl)borane in a nonpolar solvent.
 7. A process of preparing an organo-Lewis acid including the step of reacting a boron trihalide with perfluorobiphenyllithium at a temperature from −78° C. to room temperature.
 8. A process of claim 7 wherein the boron trihalide is boron trichloride.
 9. A process of claim 7 wherein the boron trihalide is added to a solution of the perfluorobiphenyllithium.
 10. A process of claim 9 wherein the boron trihalide is boron trichloride.
 11. A process of claim 10 wherein the boron trichloride is added to a solution of the perfluorobiphenyllithium which initially is at about −78° C.
 12. A method of preparing a polyolefin polymer, which method comprises polymerizing the alpha olefin using as catalyst a catalytic complex selected from the group consisting of: A) a complex of the formula [CpCp′MMe(μ-Me)MeMCpCp′]^(⊕)[MeBR′R″₂]^(⊖) B) a complex of the formula [CpCp′MH(μ-H)HMCpCp′]^(⊕)[MeBR′R″₂]^(⊖) C) a complex of the formula [CpMMe₂]^(⊕)[MeBR′R″₂]^(⊖) D) a complex of the formula [C₅H_(m)R_(4-m)XNRMMe]^(⊕)[MeBR′R″₂]^(⊖) E) a complex of the formula [CpCp′MMe]^(⊕)[MeBR′R″₂]^(⊖) where: Cp and Cp′ each is C₅H_(n)R_(5-n) where n is 0-5, or indenyl; R is alkyl or benzyl or aryl or silyl, each of 20 or less carbon atoms; M is Th, Zr, Hf, or Ti; Me is methyl; B is boron; H is hydrogen; R′ is fluorinated biphenyl; R″ is a fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused ring group; m is 0-4; X is R′″₂Si, R′″ being alkyl or aryl, either one having 10 or less carbon atoms; and N is nitrogen.
 13. The method of claim 12 wherein R′ is nonafluorobiphenyl, and wherein each R″ is nonafluorobiphenyl.
 14. The method of claim 12 wherein said alpha olefin is ethylene or propylene.
 15. The method of claim 12 wherein said catalytic complex is a complex of the formula [CpCp′MMe(μ-Me)MeMCpCp′]^(⊕)[MeBR′R″₂]^(⊖) where each of Cp and Cp′ is C₅H_(n)R_(5-n) and n is 0-5; R is alkyl or benzyl or aryl, each of 20 or less carbon atoms; M is Th, Zr, Hf, or Ti; Me is methyl; B is boron; R′ is fluorinated biphenyl; and R″ is a fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused ring group.
 16. The method of claim 15 wherein R′ is nonafluorobiphenyl, and wherein each R″ is nonafluorobiphenyl.
 17. The method of claim 15 wherein said alpha olfein is ethylene or propylene.
 18. The method of claim 15 wherein the catalytic complex used is: a) [(η⁵-C₅H₅)₂ZrMe(μ-Me)MeZr(η⁵-C₅H₅)₂]^(⊕)[MePBB]^(⊕); b) [(η⁵-1,2-Me₂C₅H₃)₂ZrMe(μ-Me)MeZr(η⁵-1,2-Me₂C₅H₃)₂]^(⊕)[MePBB]^(⊖); or c) [(η⁵-Me₅C₅)₂ZrMe(μ-Me)MeZr(η⁵-Me₅C₅)₂]^(⊕)[MePBB]^(⊖).
 19. The method of claim 12 wherein said catalytic complex is a complex of the formula [CpCp′MH(μ-H)HMCpCp′]^(⊕)[MeBR′R″₂]^(⊖) where each of Cp and Cp′ is C₅H_(n)R_(5-n) and n is 0-5; R is alkyl or benzyl or aryl, each of 20 or less carbon atoms; M is a Th, Zr, Hf, or Ti; H is a hydrogen atom; B is boron; R′ is fluorinated biphenyl; and R″ is a fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused ring group.
 20. The method of claim 19 wherein R′ is nonafluorobiphenyl, and wherein each R″ is nonafluorobiphenyl.
 21. The method of claim 19 wherein said alpha olefin is ethylene or propylene.
 22. The method of claim 19 wherein the catalytic complex used is: a) [(η⁵-C₅H₅)₂ZrH(μ-H)HZr(η⁵-C₅H₅)₂]^(⊕)[MePBB]^(⊖) or b) [(η⁵-1,2-Me₂C₅H₃)₂ZrH(μ-H)HZr(η⁵-1,2-Me₂C₅H₃)₂]^(⊕)[MePBB]^(⊖).
 23. The method of claim 12 wherein said catalytic complex is a complex of the formula [CpMMe₂]^(⊕)[MeBR′R″₂]^(⊖) where Cp is C₅H_(n)R_(5-n) and n is 0-5; R is alkyl, benzyl or aryl, each of 20 or less carbon atoms; M is Th, Zr, Hf, or Ti; Me is methyl; B is boron; R′ is fluorinated biphenyl; and R″ is a fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused ring group.
 24. The method of claim 23 wherein R′ is nonafluorobiphenyl, and wherein each R″ is nonafluorobiphenyl.
 25. The method of claim 23 wherein said alpha olfein is styrene or ethylene and 1-hexane.
 26. The method of claim 23 wherein the catalytic complex used is: a) [(η⁵-C₅H₅)TiMe₂]^(⊕)[MePBB]^(⊖); b) [(η⁵-C₅H₅)ZrMe₂]^(⊕)[MePBB]^(⊖); c) [(η⁵-C₅H₅)HfMe₂]^(⊕)[MePBB]^(⊖); d) [(η⁵-Me₂C₅H₃)HfMe₂]^(⊕)[MePBB]^(⊖); e) [(η⁵-Me₂C₅H₃)TiMe₂]^(⊕)[MePBB]^(⊖); f) [(η⁵-Me₅C₅)ZrMe₂]^(⊕)[MePBB]^(⊖); or g) [(η⁵-Me₅C₅)HfMe₂]^(⊕)[MePBB]^(⊖).
 27. The method of claim 12 wherein said catalytic complex is a complex of the formula [C₅H_(m)R_(4-m)XNRMMe]^(⊕)[MeBR′R″₂]^(⊖) where m is 0-4; R is alkyl, benzyl or aryl, each of 20 or less carbon atoms; X is R′″₂Si, R′″ being alkyl or aryl, either one having 10 or less carbon atoms; M is Th, Zr, Hf, or Ti; N is nitrogen; Me is methyl; B is boron; R′ is fluorinated biphenyl; and R″ is a fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused ring group.
 28. The method of claim 27 wherein R′ is nonafluorobiphenyl, and wherein each R″ is nonafluorobiphenyl.
 29. The method of claim 27 wherein M is Zr.
 30. The method of claim 27 wherein M is Ti.
 31. The method of claim 27 wherein said alpha olefin is ethylene or ethylene and 1-hexene.
 32. The method of claim 27 wherein the catalytic complex used is: a) [Me₂Si(^(t)BuN)(C₅Me₄)ZrMe]^(⊕)[MeBR′R″₂]^(⊖); or b) [Me₂Si(^(t)BuN)(C₅Me₄)TiMe]^(⊕)[MeBR′R″₂]^(⊖).
 33. The method of claim 12 wherein said catalytic complex is a complex of the formula [CpCp′MMe]^(⊕)[MeBR′R″₂]^(⊖) where each of Cp and Cp′ is C₅H_(n)R_(5-n) and n is 0-5, or indenyl; R is alkyl or benzyl or aryl, each of 20 or less carbon atoms; M is Th, Zr, Hf, or Ti; Me is methyl; B is boron; R′ is fluorinated biphenyl; and R″ is a fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused ring group.
 34. The method of claim 33 wherein R′ is nonafluorobiphenyl, and wherein each R″ is nonafluorobiphenyl.
 35. The method of claim 33 wherein said alpha olefin is propylene.
 36. The method of claim 33 wherein the catalytic complex used is: [(η⁵-C₅H₅)₂ZrMe]^(⊕)[MePBB]^(⊖).
 37. A method of preparing polytetrahydrofuran, which method comprises polymerizing tetrahydrofuran using as catalyst a catalytic complex of the formula [CpCp′MMe(μ-Me)MeMCpCp′]^(⊕)[MeBR′R″₂]^(⊖) where each of Cp and Cp′ is C₅H_(n)R_(5-n) and n is 0-5; R is alkyl or benzyl or aryl, each of 20 or less carbon atoms; M is Th, Zr, Hf, or Ti; Me is methyl; B is boron; R′ is fluorinated biphenyl; and R″ is a fluorinated phenyl, fluorinated biphenyl, or fluorinated polycyclic fused ring group.
 38. The method of claim 37 wherein M is zirconium.
 39. The method of claim 37 wherein the catalytic complex used is [(C₅H₃Me₂)₂(Me)Zr—Me—Zr(Me)(C₅H₃Me₂)₂]^(⊕)(MePBB)^(⊖). 